Operating Systems – OS Architecture Models ECE 344 ECE 344 Operating Systems OS Architecture • Designs that have been tried in practice – Monolithic systems – Layered systems – Virtual machines – Client/server a.k.a. Microkernels • Many of the concepts governing these architectures apply to software architectures in general ECE 344 Operating Systems Monolithic Systems • A.k.a., “The Big Mess” or spaghetti code • Prominent in the early days • The structure consists of no-structure • The system is a collection of procedures • Each procedure can call any other procedure • No information hiding (as opposed to modules, packages, classes) ECE 344 Operating Systems Monolithic Systems (cont.’d.) •A little structure, imposed by exposing a set of system calls to the outside • Supporting these system calls through utility procedures (check data passed to system call, move data around …) 1. a main procedure requesting the services basic 2. a set of service procedures that carry out system calls } structure 3. a set of utility procedures supporting the system calls system calls … service procedures … (e.g. fetching data from user utility procedures space) ECE 344 Operating Systems A Monolithic OS ECE 344 Operating Systems Pros & Cons • Tightly integrated code in one address space • Unreliable, as a bug anywhere in the kernel can bring down the whole system • Tight integration has high potential for efficient use of resources and for efficient code • Early designs lacked potential for extension • Modern designs can load executable modules dynamically (i.e., extensible) – E.g., Linux, FreeBSD, Solaris ECE 344 Operating Systems MS-DOS System Structure • MS-DOS – written to provide the most functionality in the least space – not divided into modules – although MS-DOS has some structure, its interfaces and levels of functionality are not well separated ECE 344 Operating Systems MS-DOS Structure ECE 344 Operating Systems UNIX System Structure • UNIX – limited by hardware functionality (at its time of origin), the original UNIX operating system had limited structuring. • The UNIX OS consists of two separable parts. – Systems programs – The kernel • Consists of everything below the system-call interface and above the physical hardware •Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level. ECE 344 Operating Systems UNIX System Structure ECE 344 Operating Systems Layered Systems • Generalization of previous scheme • Organization into a hierarchy of layers • Layer n+1 uses services (exclusively) supported by layer n • Easier to extend and evolve • A call may have to propagate through lots of layers – At occasions (optimization) layer n+1 may also access layers n-k directly • Upcall, layer n-k calls into layer n has also been proposed (e.g., in the context of thread scheduling) ECE 344 Operating Systems Operating System Layers ECE 344 Operating Systems THE (Dijkstra,1968) Operator User programs Input/output mgmt. Operator-process com. Memory mgmt. Processor allocation and multiprog. ECE 344 Operating Systems Ring-based protection Increasing privilege level System calls ECE 344 Operating Systems OS/2 Layer Structure ECE 344 Operating Systems Microkernel System Structure • Moves as much as possible from the kernel into “user”space • Communication takes place between user modules using message passing • Benefits: - easier to extend a microkernel -easier to port the operating system to new architectures - more reliable (less code is running in kernel mode) - more secure (a server crashing in userspace) • Not clear what should go into the microkernel • Mach, QNX, NT, L4 ECE 344 Operating Systems Microkernel ECE 344 Operating Systems Microkernel Examples •AIX •MERT •AmigaOS •Minix •Amoeba •MorphOS • Chorus microkernel •NewOS •EROS •QNX •Haiku • Phoenix-RTOS •K42 • RadiOS • LSE/OS (a nanokernel) • Spring operating system • KeyKOS (a nanokernel) •VSTa • The L4 microkernel family • Symbian OS • Mach, used in GNU Hurd, •OSE • NEXTSTEP, OPENSTEP, and Mac OS X Nanokernel is a very light-weight microkernel ECE 344 Operating Systems Windows NT Client-Server Structure ECE 344 Operating Systems Monolithic vs. Microkernel • Mon. tend to be easier to design, therefore faster development cycle and more potential for growth (see Linux) • Mon. tend to be more efficient due to use of shared kernel memory (instead of IPC) – However, very efficient micro kernels have been designed in research and laboratory settings • Micro. tend to be used for embedded systems (e.g., robotic, medical etc.) •In Micro. Many OS components reside in their own, private protected address space (not possibly in Mon. designs) ECE 344 Operating Systems Others • Hybrid kernels (a.k.a. modified microkernel) – Add more code to the kernel for efficiency • Windows 2000, Windows XP – Based on message passing – Based on minimalist concept – Not based on loading modules ECE 344 Operating Systems Exokernel • Traditionally – Kernel hides hardware from application – Based on conceptual model (files systems, virtual address space, schedulers, sockets) • Generally, this eases application development • Sometimes problematic (e.g., security / privacy of stored data upon deletion) – Security-oriented application requires file system to delete data – Reliability-oriented application requires file system to keep data for failure recovery purposes ECE 344 Operating Systems Exokernel cont’d. • Kernel allocates physical resources to application • Application decides what to do with the resources – Application can link to a libOS to emulate a conventional OS – Application uses resources as it wishes • Exokernel could emulate several OSes, one for each application it runs ECE 344 Operating Systems Exokernel Summary • Provide as few abstractions as possible • Kernel is relatively small – Allocates, protects, and multiplexes resources • Low-level hardware access enables to build custom abstractions on per application basis – Performance reasons • Does not force the layering of abstractions ECE 344 Operating Systems Virtual Machines •A virtual machine takes the layered approach to its logical conclusion. Hardware is simulated in software; all resources are virtualized; individual OS run on virtualized resources • A virtual machine provides an interface identical to the underlying bare hardware • The operating system creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory ECE 344 Operating Systems Virtual Machines (Cont.) • The resources of the physical computer are shared to create the virtual machines. –CPU scheduling can create the appearance that users have their own processor. – Spooling and a file system can provide virtual disks, virtual memory and virtual printers. – A normal user time-sharing terminal serves as the virtual machine operator’s console ECE 344 Operating Systems System Models Non-virtual Machine Virtual Machine ECE 344 Operating Systems Pros/Cons of Virtual Machines • VM model provides complete protection • At the cost of not enabling any direct resource sharing • A virtual-machine system is a perfect vehicle for operating-systems research and development. – System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation. • The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine. • Available in practice (IBM, VMWare, Sys161) ECE 344 Operating Systems.